I stopped speaking on my 27th birthday in 1973, because I found myself arguing all the time. After witnessing an oil spill in San Francisco Bay in 1971, I gave up using motorised vehicles and started walking everywhere as a statement about pollution. I lived in a small village on the west coast of America, where I kept getting into debates about whether one person could make a difference. I would rant and rave about how everyone should do what I was doing.

I used words to hide from other people, and from myself. I made things up: if someone told me they were going on a trip to Europe, I’d pretend I was going, too; I guess I had low self-esteem. I decided not to speak for one day, as a kind of gift to my community. My girlfriend thought I was doing a nice thing. When I woke the next day, I didn’t see any reason to speak, so I didn’t. When others spoke to me, I mimed that I was being silent. They were thrilled.

For the first week, everyone thought it was fun, and I realised I was actually listening for a change. I had always been so used to thinking about what I was going to say next.

After a few weeks, people started getting worried. My girlfriend wanted me to stop. I sent a letter to my parents in Philadelphia, telling them I hadn’t spoken for three weeks and that I was thinking of not speaking for a year. My dad got on the next plane. They thought I’d been taken over by a Californian cult.

When he arrived, he told me to get in the car to go to his hotel, but I shook my head and walked my fingers in the air. When we got into his hotel room, he said, “OK, you can talk now.” But when I still wouldn’t, he was beside himself. This went on for years.

I liked not speaking. It gave me peace. People ask if I spoke to myself, but why would I? I’d just have complained when I had no one to blame but myself. I did slip up a couple of times. After about six months, I bumped into someone in the grocery store and said, “Excuse me.” My laugh was completely silent; friends said that was the weirdest thing. My girlfriend came around to understanding why I was doing it, but when I decided to walk from California to Oregon to explore the wilderness, we went our separate ways.

On the 10th anniversary of not speaking, I spoke for a day. I didn’t want it to feel as if it wasn’t a choice. I was in California and I called my parents. My mother picked up and thought it was my brother messing around. I had to prove it was me by telling her something only we knew: I reminded her of when I walked to meet her and my dad in San Francisco and we were in an elevator and she said, “If you were really serious about this walking, you wouldn’t ride in an elevator.”

I spent 28 years on death row

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During this time, I gained a bachelor’s, master’s and PhD in environmental studies. I learned some sign language to help me communicate, and I walked across the United States, playing my banjo, and became known as Planetwalker.

After 17 years of not speaking, I felt I had something to say. People came to hear me in a hotel in Washington DC. My first words were, “Thank you for being here.” But I didn’t recognise my voice and just started laughing. I saw my dad sitting in the audience, looking at me like, “Yes, he really is crazy.” Having listened to thousands of people, I realised we had a narrow view of what the environment is. It’s more than saving trees; it’s about how we treat each other, and that includes gender and economic equality and civil rights.

I met my wife just after I started speaking again and, because my PhD focused on oil spills, the US Coastguard hired me after the Exxon Valdez disaster. Now I teach in schools and give talks around the world. I started using vehicles again in 1995, after 22 years, when, while walking through Venezuela to Brazil, I realised that walking had become a prison for me. I still practise being silent every morning, and sometimes don’t speak for several days at a time. It reminds me to listen properly; not to judge what I think I’m hearing, but to try to understand what people are really saying.